VIENNA — Concerns are growing that Al Qaeda or a related group could detonate a "dirty bomb" that would spew radioactive fallout across an American or European city, according to intelligence analysts, diplomats and independent nuclear experts.
Although safeguards protecting nuclear weapons and their components have improved, experts said the radioactive materials that wrap around conventional explosives to create a contaminating bomb remained available worldwide -- and were often stored in non-secure locations.
Detonating a dirty bomb would not cause the death and devastation wrought by a nuclear weapon, but officials and counter-terrorism experts predicted that it would result in some fatalities, radiation sickness, mass panic and enormous economic damage.
Intelligence agencies have reported no reliable, specific threats involving dirty bombs or nuclear weapons, but senior U.S. and European officials and outside experts said several factors had heightened fears in recent weeks.
They said concerns were focused on three Al Qaeda operatives who led experiments involving dirty bombs and chemical weapons and on widely held suspicions that a special wing of the terrorist network was planning a spectacular attack.
They also said that chatter justifying the use of nuclear weapons against the U.S. had increased on radical Islamic websites as the occupation of Iraq stretches into its second year.
One focus of anxiety is the Athens Olympic Games in August. Recent security exercises there concentrated on mock attacks involving a dirty bomb, a chemical explosion and a hijacked jetliner.
Another potential target is the North Atlantic Treaty Organization summit scheduled for June in Istanbul, Turkey, which will be attended by President Bush. The threat was underlined by Turkey's disclosure Monday that it had arrested members of a group linked to Al Qaeda who reportedly planned to bomb the summit.
The threat of attack is great enough that a senior European intelligence official, speaking on condition of anonymity, said it is "not a matter of if there is a nuclear-related attack by Al Qaeda, but when it occurs."
The warning echoed remarks made last June by Eliza Manningham-Buller, director of Britain's domestic intelligence service, MI5. She said renegade scientists have aided Al Qaeda's efforts to develop chemical, biological, radiological and nuclear weapons, sometimes referred to as CBRN.
